Ideal Solutions—Raoults Law

The concept of an ideal solution is important in the development of an understanding of the properties of real solutions. In a liquid solution, molecules are in intimate contact with one another so that the question of ideality is determined by the nature of the intermolecular forces. Suppose a solution is formed by mixing two liquids, A and B. Then, the solution is ideal if the intermolecular forces between A and B molecules are no different from those between A and A, or B and B molecules. 

An indication of whether or not the above condition for ideality is met is obtained from the vapor pressure of the solution. At a given temperature, the vapor pressure of a pure liquid is a measure of the ability of molecules to escape from the liquid to the gas phase. By studying the vapor pressure of a solution as a function of its composition at constant temperature one may assess the solution s ideality or its degree of departure from ideality. For an ideal solution, the tendency of molecule A to escape is proportional to its mole fraction, that is, to its concentration expressed in terms of the fraction of molecules which are of type A. The proportionality constant must be the vapor pressure of pure component A because this vapor pressure is reached when the mole fraction is unity. This result is Raoult s law, which is expressed mathematically as 

A system which is close to ideal in its behavior is a solution of benzene and toluene. These molecules are very similar in structure so that the intermolecular forces between benzene and toluene do not dilfer greatly from those between benzene molecules or toluene molecules in each pure liquid. The vapor pressure diagram for this system at 25°C is shown in fig. 1.4. Since toluene has a higher molecular weight, its vapor pressure as a pure liquid is lower (3.25 kPa) than that of benzene (12.69 kPa). The total vapor pressure Pj is obtained by adding those of components A and B, so that 

The total vapor pressure also a linear function of the mole fractions Aa and Ag. 

It is clear from the above that Raoult s law defines ideality by relating the properties of the liquid solution to the vapor with which it is in equilibrium. 

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